My external sensor doesn't appear when I plug it in, or it disappears after a while.

If your external sensor never shows up when you plug it into
your working Twine, here are a few things to check:

Are you in "fast" mode? Normally, Twine updates about once a
minute, so you may not see your sensor pop up right away. Turn on
fast mode by going to the rule editing page and flipping your Twine
onto its back (after which you can turn it back up). Then, you'll
get updates from Twine in real time.

Make sure you've pressed in the sensor cable all the way. The
last bit can be harder.

If your sensor shows up at first but disappears after a while,
this is fixed in a software update. Resave your rules, even if
they're empty, to get the update.